We have recently shown that specialized myocytes of the rabbit heart express a cytoskeletal protein similar to the M subunit of neurofilaments (NF). Since this result was obtained using a single anti-NF-M monoclonal antibody, we tested on conduction myocytes a panel of five anti-NF antibodies, specific for each of the three NF subunits and for phosphorylated and non-phosphorylated epitopes. Two antibodies, one specific for the L subunit and one for phosphorylated M subunit of NF, reacted with specialized myocytes in immunohistochemistry. In immunoblots on conduction tissue homogenates the two antibodies recognized two polypeptides with electrophoretic mobility and solubility properties identical to those of NF-L and NF-M in the sciatic nerve. The subcellular distribution of NF immunoreactivity in specialized myocytes was very similar to desmin localization; namely, it was distributed on large filamentous bundles and on fine filaments localized transversely at the level of the Z line. At the ultrastructural level, immunoreactive filaments were localized in the intermyofibrillar space and connected myofibrils with mitochondria. Co-expression of NF proteins and desmin was also observed in vitro in a minor population of cardiac myocytes cultured from embryonic rabbit heart. In most cases NF immunoreactivity co-localized with desmin, especially where filaments were well organized, but in some cells anti-NF and anti-desmin antibodies labelled different filamentous structures. These results indicate that NF proteins are structural components of the cytoskeleton of specialized myocytes and show a subcellular distribution very similar to desmin. Such a composition of intermediate filaments indicates that in these cardiac cells muscle differentiation is compatible with the expression of neuronal proteins.